This invention relates generally to conical cutters (usually called cones) used in roller bits employed in oil-well drilling and in drilling of holes for mining purposes. The invention further concerns a process through which the conical cutters may be most conveniently manufactured as integrated composite structures, and secondly, novel cutters and cutter component structures as well as composition thereof provide important properties associated with localized sections of the cutters.
Conical cutters must operate under severe environmental conditions and withstand a variety of "bit-life" reducing interactions with the immediate surroundings. These include abrasive or erosive actions of the rock being drilled, impact, compressive and vibrational forces that result from rotation of the bit under the weight put on the bit, and the sliding wear and impact actions of the journal pin around which the cone is rotating. The severity, as well as the variety of life-reducing forces acting upon conical cutters, dictate that these cutters not be made of a simple material of uniform properties if they are to provide a cost-effective, down-hole service life. Instead, localized properties of cone sections should withstand the localized forces acting on those sections.
Conventional cones utilizing tungsten carbide inserts (TCI) are commonly manufactured from a forged shape. Holes are drilled circumferentially around the forged cutter body to receive hard-cutting elements, such as cobalt cemented tungsten carbide inserts or TCI's, which are press-fitted into the holes. TCI shape must, therefore, be the same as the hole shape, and have parallel side surfaces.
The cone body normally requires surface hardening to withstand the erosive/abrasive effect of rock drilling. This may be accomplished by any of the widely used surface modification or coating techniques, such as transformation hardening, carburizing, nitriding, hard-facing, hard metal coating or brazed-on hard metal cladding.
In addition, interior surfaces of the cone are required in certain areas to be hard, wear and impact resistant to accommodate loading from both the thrust and the radial directions (with respect to the journal pin axial direction). Consequently, these surfaces are also hardened by a surface hardening process. On the journal side, the pin surfaces likely to contact "thrust bearing" surfaces are usually hardfaced and run against a hardened cone or a hardened nose button insert in the cone or a carburized tool steel bushing. In most roller cones, a row of uncapped balls run in races between the nose pin and the roller or journal bearing. These balls may carry some thrust loading, but their primary function is to retain the cone on the journal pin when not pressing against the bottom of the hole.
The major load is the radial load and is carried substantially either by a full complement of cylindrical rollers used primarily in mining operations, or a sealed journal bearing used in oil-field drilling. The journal bearings are normally operated with grease lubrication and employ additional support to prolong bearing life; i.e., self-lubricating porous floating rings.sup.(1), beryllium-copper alloy bearing coated with a soft metal lubricating film.sup.(2,3), a bearing with inlays of soft metal to provide lubrication and heat transfer.sup.(4), or an aluminum bronze inlay.sup.(5) in the cone as the soft, lubricating member of the journal-cone bearing couple.